http://www.boards.co.uk/equipment/finsize_calc.asp
Some of the fin companys have detailed sail ranges for their fins, others none. The companies taht offer great support are True Ames, GSport, Maui Fin , Select, Tarifa, FinWorks, I like the look of Nik Bakers line and Kevin Prichard has his MFC for sale on his blog at a good price delivered, most of the shops have knowledgeable staff reference size, Gorge Surf Shop, Windance, Big Winds and DELTA Windsurf , all US shops great staff.